Process for preparing high strength paper

ABSTRACT

The present invention provides a process for the preparation of an aqueous composition comprising polysilicate coated mineral particles, wherein the process comprises subjecting an aqueous composition comprising gelled polysilicate and a mineral to shear of at least 5000 rpm for at least 30 seconds. The present invention also provides polysilicate coated mineral particles obtainable by this process as well as paper filled with these particles.

The present invention refers to polysilicate coated mineral particles, which are suitable for use as filler in paper, to an aqueous composition comprising these particles, to a process for the preparation of this aqueous composition, to paper filled with these particles and to a process for the preparation of filled paper.

Generally fillers, which are usually mineral materials, are included into paper in order to reduce the amount of fibres and thus costs. However, the paper strength usually diminishes as the level of filler is increased.

Thus, there is an ongoing search for paper having high filler content and at the same time acceptable paper strength.

U.S. Pat. No. 6,623,555 describes a method of making a composite pigment of precipitated calcium carbonate and a silicon compound, for use, for example, as a filler for paper making or as coating pigment. The method includes the steps of introducing a soluble silicate compound into an aqueous medium containing a precipitate of calcium carbonate and precipitating an insoluble silicon compound upon the precipitated calcium carbonate by carbonation of the reaction mixture. The temperature of the reaction mixture during deposition of the silicon compound upon the precipitated calcium carbonate is at least 50° C. The obtained aqueous composition containing the formed composite pigment is of low viscosity. The disadvantage of the composite pigment of U.S. Pat. No. 6,623,555 is that it yields filled paper of low strength when used as filler.

WO 00/26305 describes a method for preparing an aqueous composition comprising partly silica coated calcium carbonate particles suitable as filler for paper. The method includes mixing calcium carbonate with about 3.5% by weight soluble silicate based on the weight of calcium carbonate and allowing the silicate slowly to react at room temperature without pH adjustment to form an insoluble silica coating. The disadvantage of this method is that the reaction time is very long, for example 50 hours, thus rendering the process not technically feasible from an industrial point of view. In addition, the amount of silica in the silica coated filler is very low, and the silica coated filler yields paper of a similar strength to paper filled with standard calcium carbonates.

WO 95/03251 describes a method for preparing a mixed pigment of calcium carbonate and silica, useful as filler for paper. The method involves admixing lime milk and an aqueous sodium silicate solution and raising the temperature of the mixture to 55 to 170° C. and thereafter supplying carbon dioxide to the mixture until the pH of the mixture falls to 7 or below, thus precipitating a mixed pigment comprising calcium carbonate and silica. The ratio of silica/CaO is 3.6/1. Thus, the ratio of silica/CaCO₃ in the obtained mixed pigment is 2/1 (or 66/33) and thus very high.

EP 356 406 A1 describes a process for the preparation of calcium carbonate particles with an acid-resistant coating, in which process a slurry of calcium carbonate particles is mixed simultaneously with the solution of a zinc compound and a solution of a silica-containing substance at a temperature of 70 to 95° C. at pH 8 to 11.

U.S. Pat. No. 5,164,006 describes a method for preparing an acid resistant calcium carbonate pigment, which method includes mixing a sodium silicate solution with a calcium carbonarte slurry at 75 to 80° C., adjusting the pH to 10.2 to 10.7 by adding carbon dioxide, cooling the reaction mixture and adding zinc chloride.

The disadvantage of the methods of EP 356 406 A1 and U.S. Pat. No. 5,164,006 is that the methods involve the additional step of adding a zinc compound.

It is an object of the present invention to provide filled paper or paper board of improved dry strength, in particular of improved tensile strength and internal bond strength, with respect to the amount of filler.

This object is solved by the process of claim 1, the composition of claim 9, the polysilicate coated mineral particles of claim 15, the paper of claim 16 and the process of claim 17.

Part of the invention is a process for the preparation of an aqueous composition comprising polysilicate coated mineral particles, wherein the process comprises the step of subjecting an aqueous composition comprising gelled polysilicate and mineral to shear of at least 5,000 rpm for at least 30 seconds.

Preferably, the aqueous composition comprising gelled polysilicate and mineral is subjected to shear of at least 8,000 rpm, more preferably to shear in the range of 10,000 to 30,000 rpm. Preferably, the shear is applied for at least 1 minute. The shear can be applied for any time. For practical reasons, however, shear is usually applied for maximum 1 hour, preferably maximum 30 minutes, more preferably maximum 15 minutes.

Preferably, the aqueous composition comprising polysilicate coated mineral particles is characterized by a viscosity of at least 500 mPas (when measured at a concentration of 12% by dry weight polysilicate coated mineral particles based on the weight of the composition, using a Brookfield viscometer at 25° C. and 100 rpm, spindle 4, and the result is taken 30 seconds after the start of the test).

Preferably, the aqueous composition comprising gelled polysilicate and mineral is obtainable by polymerization of a silicate in the presence of a mineral.

Usually, the silicate is water soluble. Preferably the silicate is an alkaline earth metal silicate such as magnesium silicate or an alkali metal silicate such as lithium, portassium or sodium silicate. More preferably it is an alkali metal silicate; most preferably it is sodium silicate. Preferred sodium silicate has a weight ratio of Na₂O to SiO₂ is in the range of 2:1 to 1:4 more preferably it is in the range of 1:2 to 1:4, most preferably in the range of 1:2.5 to 1:3.5. Preferably, the silicate is employed in form of an aqueous solution.

Examples of minerals are titanium dioxide, aluminium trihydrate, mineral silicates such as talc, mica, zeolite, clay, e.g. kaolin, and calcinated clay, precipitated silicates and calcium carbonates such as ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC). A preferred mineral is calcium carbonate. The mineral is usually substantially not water-soluble under the conditions of polymerization. The mineral can be employed in solid form or as aqueous slurry.

The mineral can be already present at the start of the polymerization of the silicate or it can be added during polymerization of the silicate.

During polymerization of the silicate gellation occurs. Preferably, the polymerization of the silicate is performed under stirring. Once polymerization and thus gellation is substantially completed, the intensity of the stirring is preferably increased to achieve an aqueous composition comprising gelled polysilicate and mineral of relatively low viscosity, for example below 300 mPas (when measured at a concentration of 12% by dry weight gelled polysilicate and mineral, using a Brookfield viscometer at 25° C. and 100 rpm, spindle 4, and the result is taken 30 seconds after the start of the test). This aqueous composition comprising gelled polysilicate and mineral of relatively low viscosity can then be applied to the step of high shear (at least 5′000 rpm, for 30 seconds).

The polymerization of the silicate can be initiated by adjusting the pH to a pH of below 10 by addition of an acid. Examples of acids are carbon dioxide, mineral acids such as hydrochloric acid or sulfuric acid, organic acids such as acetic acid and alkali metal borates, aluminates or stannates. Preferably, carbon dioxide or mineral acids are used as acid.

In the first embodiment of the process, where the mineral is already present at the start of the polymerization, the polymerization of the silicate is preferably initiated using carbon dioxide, and the pH is preferably adjusted to a pH in the range of 5 to 9, more preferably about 7.

In the second embodiment of the process, where the mineral is added during polymerization of the silicate, the polymerization of the silicate is preferably initiated using a mineral acid, and the pH is preferably adjusted to a pH in the range of 2 to 10.5, more preferably of 7 to 9.

If in the second embodiment of the process a mineral is used, which can dissolve at acidic pH, for example calcium carbonate, it is preferred that the pH is adjusted to a pH in the range of 8.5 to 10.

Preferably, in both embodiments, the polymerization of the silicate is performed at a temperature of below 50° C. More preferably, it is performed at a temperature in the range of 15 to 35° C. Most preferably, it is performed at a temperature in the range of 20 to 30° C.

Usually the polymerization of the silicate is substantially complete within 12 hours after start of the polymerization. Preferably, it is complete within 5 hours, more preferably within 2 hour and most preferably within 1 hour.

The amount of mineral is preferably in the range of 5 to 20%, more preferably of 8 to 12%, by weight based on the weight of aqueous composition comprising gelled polysilicate and mineral.

The amount of silicate is preferably in the range of 0.01 to 10%, more preferably of 0.5 to 4%, more preferably of 1 to 3% by weight, based on the weight of the aqueous composition comprising gelled polysilicate and mineral.

Preferably, the amount of gelled polysilicate and mineral is in the range of 1 to 30%, more preferably 5 to 15%, most preferably 10 to 14%, by dry weight based on the weight of the aqueous composition comprising the gelled polysilicate and mineral.

Also part of the invention is an aqueous composition comprising polysilicate coated mineral particles, which is obtainable by the process of the present invention.

Preferably, the aqueous composition comprising polysilicate coated mineral particles is characterized by a viscosity of at least 500 mPas (when measured at a concentration of 12% by dry weight polysilicate coated mineral particles based on the weight of the composition, using a Brookfield viscometer at 25° C. and 100 rpm, spindle 4, and the result is taken 30 seconds after the start of the test).

Preferably, the polysilicate coated mineral particles are not completely coated with the polysilicate. For example, a polysilicate coated calcium carbonate particle dissolves at a pH of below 6.

The amount of mineral can be between 40 and 99% by weight based on the weight of the polysilicate coated mineral particle. Preferably it is between 60 and 95% by weight, more preferably it is between 80 and 90% by weight, most preferably it is between 78 and 88% by weight.

The amount of polysilicate can be between 1 and 60% by weight based on the weight of the mineral; preferably it is between 5 and 40% by weight, more preferably it is between 15 and 25% by weight.

The size of the polysilicate coated mineral particles can be between 1 and 1000 μm, preferably it is between 8 and 30 μm, more preferably, it is between 10 to 27 μm.

Preferably, the amount of polysilicate coated mineral particles is between 1 and 30% by weight based on the weight of the aqueous composition comprising the polysilicate coated mineral particles. More preferably, it is between 10 and 15% by weight.

Also part of the present invention is a polysilicate coated mineral particle obtainable from the aqueous composition of the present invention by removal of water. Removal of water can be performed by any suitable method such as filtration, decantation or distillation or any suitable combination of methods.

Another part of the present invention is paper or paper board filled with the polysilicate coated mineral particles of the present invention.

Preferably, the weight ratio of fibre/polysilicate coated mineral particles in the filled paper or paper board is in the range of 90/10 to 30/70, preferably it is in the range of 80/20 to 60/40, and more preferably it is in the range of 75/25 to 65/35.

Also part of the invention is a process for preparing the filled paper or paper board of the present invention, which process comprises the step of adding the polysilicate coated mineral particles or the aquepous composition comprising the polysilicate coated mineral particles to a cellulosic suspension prior to drainage of the cellulosic suspension on a wire, where a web is formed, which is subsequently dried.

Preferably, filled paper is prepared.

The polysilicate coated mineral particles of the present invention can be used in conjunction with standard wet end additives such as cationic coagulants, dry strength agents, retention agents, sizing agents and optical brighteners. It is also possible that it is used together with other fillers such as calcium carbonate, although this is not particulary preferred.

The polysilicate coated mineral particles can be added to the thick stock, the thin stick or to the white water.

Also part of the invention is a method for improving the tensile strength and internal bond strength of filled paper or paper board with respect to the basis weight of the filled paper or paper board, which method involves adding the polysilicate coated mineral particles or the aqueous composition comprising the polysilicate coated mineral particles of the present invention to a cellulosic suspension prior to drainage of the cellulosic suspension on a wire, where a web is formed, which is subsequently dried.

The polysilicate coated mineral particles of the present invention have the advantage that when used as filler for paper, the ratio of tensile strength (breaking length) and Internal bonding strength (Scott bond strength)/ash content of filled paper is improved. In other words, the use of polysilicate coated mineral as filler leads either to filled paper of increased tensile strength and internal bonding strength for a given ash content, or to paper of lower ash content for a given tensile strength or Internal bonding strength. At the same time, the paper shows good formation and good opacity.

FIG. 1 shows the correlation between breaking length and sheet ash content of handsheets containing Calopake®F, Calopake®F and polysilicate, and polysilicate coated Calopake®F.

FIG. 2 shows the correlation between breaking length and sheet ash content of handsheets containing Albaca®HO, Albaca®LO, Syncarb F0474, respectively, Miconapaque HB and the respective polysilicate coated calcium carbonates.

FIG. 3 shows the correlation between Scott bond strength and sheet ash content of handsheets containing Calopake®F, Calopake®F and polysilicate, and polysilicate coated Calopake®F.

FIG. 4 shows the correlation between Scott bond strength and sheet ash content of handsheets containing Albaca®HO, Albaca®LO, Syncarb F0474, respectively, Miconapaque HB and the respective polysilicate coated calcium carbonates.

EXAMPLES 1 TO 5 Preparation of Aqueous Compositions Comprising Polysilicate Coated Calcium Carbonate Particles Using Carbon Dioxide as Acid

100 g oven dry weight of calcium carbonate (see table 1) is taken and diluted to 930 g using deionised water. To this is added 70 g of sodium silicate (28.5% (w/v) SiO₂, 8.9% (w/v) Na₂O). Thus, the amount of calcium carbonate is 10% by dry weight based on the volume of the reaction mixture, and the amount of sodium silicate is 2% by dry weight based on the volume of the reaction mixture. The two aqueous materials are mixed thoroughly at room temperature using a magnetic stirrer and carbon dioxide is added using a gas diffusion tube to achieve a pH of 7.0 at room temperature. In some instances the pH of 7 is not achieved prior to gelling of the polysilicate. The reaction mixture is allowed to gel at room temperature. 20 Minutes after gellation is substantially complete the gel is broken down by the action of the magnetic stirrer and a slurry containing visible particles (>2 mm) is obtained. This slurry is then sheared using an ultra thurrax homogeniser for 1 minute at 13,500 rpm. The aqueous composition obtained contains 12% by dry weight polysilicate coated calcium carbonate based on the volume of the composition.

The calcium carbonates used are listed in table 1 along with their particle size and the particle size of the polysilicate coated calcium carbonate particles obtained. The particle sizes are analyzed using a Malvern MasterSizer 2000 Particle Size Analyser.

TABLE 1 Trade name Mean particle Mean particle of calcium size of calcium size of polysilicate carbonate carbonate before coated calcium used coating [um] carbonate [um] example 1 Calopake ® F 3.191 17.987 example 2 Albacar ® HO 2.673 13.917 example 3 Albacar ® LO 3.419 10.676 example 4 Syncarb F0474-GO 2.425 26.757 example 5 Miconapaque HB 2.649 18.988

Calopake®F is a dry precipitated calcium carbonate, Albaca®HO, Albaca®LO and Syncarb F0474-GO are dispersed precipitated calcium carbonates and Miconapaque HB is a dispersed ground calcium carbonate. Calopake®F, Albaca®HO and Albaca®LO are sold by Mineral Technologies and Miconapaque HB is sold by Columbia River Carbonates.

The viscosities of the aqueous compositions comprising the polysilicate coated calcium carbonates of examples 1, 4 and 5 are measured using a Brookfield DVII-Pro viscometer and the results are taken 30 seconds after the start of the test.

TABLE 2 Brookfield viscosity (spindle 4, 100 rpm, 25° C.) Trade name of [mPas] calcium carbonate After used Before the shear step the shear step example 1 Calopake ® F 250 596 example 4 Syncarb F0474-GO 166 960 example 5 Miconapaque HB 198 620

When trying to acidify the aqueous compositions comprising polysilicate coated calcium carbonates of examples 1 to 5 to a pH of about 1.5 using hydrochloric acid, the calcium carbonate rapidly dissolves before the pH reaches 6 indicating that the silica surface is not a complete coating providing a barrier to the acid.

COMPARATIVE EXAMPLE 1 Preparation of an Aqueous Composition Comprising a Polysilicate

An aqueous composition comprising solely polysilicate without calcium carbonate is prepared in analogy to examples 1 to 5, except that no calcium carbonate is added. The amount of sodium silicate is again 2% by weight based on the volume of the reaction mixture. The aqueous composition obtained contains 2% by dry weight polysilicate based on the volume of the composition

EXAMPLES 6 TO 10 Preparation of Handsheets Containing Polysilicate Coated Calcium Carbonates

Handsheets containing silica coated calcium carbonates are prepared as follows: A standard laboratory fine paper stock containing 0.5% by dry weight fibres (70% bleached birch fibres and 30% bleached pine fibres) based on the weight of the stock is beaten to 47° SR. The stock is stirred at 1000 rpm for 5 seconds. An aqueous solution containing 0.5% (w/v) Raisamy®50021, a cationic potato starch, is added to obtain a concentration of 5 kg/t Raisamy®50021 based on the dry weight of the final paper and stirring at 1000 rpm is continued for 30 seconds. The required amounts of the aqueous compositions of examples 1 to 5 comprising the polysilicate coated calcium carbonate particles (see table 2) are added and stirring at 1000 rpm is continued for further 30 seconds. An aqueous solution containing 0.1% (w/v) Percol 175, a cationic polyacrylamide, is added to obtain a concentration of 500 g/t Percol 175 based on the dry weight of the final paper and stirring at 1000 rpm is continued for further 30 seconds. An aqueous solution containing 0.1% (w/v) Hydrocol 2D6, a water-swellable montmorrilonite clay, is added to obtain a concentration of 2000 g/t Hydrocol 2D5 based on the dry weight of the final paper and stirring is continued for 15 seconds at 500 rpm. The treated stock is formed into sheets using a semi automatic sheet maker, pressed and dried at 95° C.

The order of addition for the preparation of handsheets of examples 6 to 10 is summarized in the diagramm 1 below:

COMPARATIVE EXAMPLES 2 TO 8 Preparation of Handsheets Containing Calcium Carbonate, but no Polysilicate Coated Calcium Carbonate

Comparative handsheets are prepared as described for the handsheets of examples 6 to 10, but without adding the polysilicate coated calcium carbonates. Instead the respective calcium carbonate is added before the addition of starch. The order of addition for the preparation of handsheets of comparative examples 2 to 8 is summarized in the diagramm 2 below:

COMPARATIVE EXAMPLE 9 Preparation of Handsheets Containing Calcium Carbonate and Polysilicate, but No Polysilicate Coated Calcium Carbonate

Handsheet of comparative example 9 is prepared as described for handsheets of examples 6 to 10, but the polysilicate of comparative example 2 is used instead of the polysilicate coated calcium carbonates. In addition, the respective calcium carbonate is added before the addition of starch. The order of addition for the preparation of handsheet of comparative example 9 is summarized in diagramm 3 below:

The weight ratios of fibre, silica coated calcium carbonate, calcium carbonate, respectively, polysilicate used in the handsheets of examples 6 to 10 and comparative examples 2 to 9 is summarized in table 2:

TABLE 2 Handsheets Dry weight Dry silica treated Dry weight Tradename of weight calcium Calcium Dry weight Calcium carbonate Fibre carbonate carbonate Polysilicate No. used [%]¹ [%]¹ [%]¹ [%]¹ comp. ex. 2 Calopake ® F 85 — 15 — comp. ex. 3 Calopake ® F 65 — 35 — comp. ex. 4 Calopake ® F 75 — 25 — comp. ex. 9 Calopake ® F 70 — 25 5 6 Calopake ® F 70 30² — — comp. ex. 5 Albacar ® HO 75 — 25 — 7 Albacar ® HO 70 30² — — comp. ex. 6 Albacar ® LO 75 — 25 — 8 Albacar ® LO 70 30² — — comp. ex. 7 Syncarb F0474-GO 75 — 25 — 9 Syncarb F0474-GO 70 30² — — comp. ex. 8 Miconapaque HB 75 — 25 — 10  Miconapaque HB 70 30² — — ¹Based on the total weight of fibre, silica treated cacium carbonate, calcium carbonate and polysilicate. ²Containing 25% by weight calcium carbonate and 5% by weight polysilicate.

Evaluation of the Handsheets of Examples 6 to 10 and Comparative Examples 2 to 9

The handsheet samples of examples 6 to 10 and comparative examples 3 to 9 are conditioned for at least 24 hours at 50% relative humidity and 23° C. according to the Tappi test method T402.

Basis weight is a function of weight and handsheet area and is calculated from the handsheet weight. Ash content is determined using a CEM microwave furnace at a temperature of 525° C. 2×15 mm strips are taken from each handsheet and are tested for tensile strength (Breaking Length) according to Tappi T494 using an EJA Single column tensile tester. Internal bond strength (Scott Bond Strength) is determined using a Scott bond Instrument in accordance with TAPPI Test Method T569

TABLE 3 Handsheet Basis Scott Bond weight Ash Content Breaking Length Strength Example No. [gm⁻²] [%] [m] [Jm⁻²] comp. ex. 2 71.0 16.7 3869.6 409 comp. ex. 3 71.4 36.1 1760.4 120 comp. ex. 4 71.1 27.9 2537.5 245 comp. ex. 9 72.2 31.4 2680.7 187 6 72.2 26.9 3356.0 298 comp. ex. 5 67.4 32.5 2226.9 187 7 71.5 39.7 2522.9 178 comp. ex. 6 73.1 33.0 2523.9 254 8 71.5 35.5 2651.3 216 comp. ex. 7 64.6 22.1 3735.1 446 9 71.8 38.2 2878.5 241 comp. ex. 8 71.8 36.4 2999.3 320 10 72.2 45.3 3257.3 289

The results for handsheets of comparative examples 2 to 4 show that the higher the fibre content and the lower the calcium carbonate content, the higher the Breaking Length and Scott Bond Strength. Replacement of part of the fibres with polysilicate while keeping the amount of calcium carbonate constant leads to an increase in Breaking Length (see results for handsheets of comparative examples 4 and 9).

The correlation of breaking length and ash content for the handsheets of example 6 and comparative examples 2 to 4 and 9 is shown in FIG. 1.

It can be assumed that the correlation of breaking length and ash content of the handsheets of example 6 and comparative example 9 behaves in analogy to the correlation of breaking length and ash content as shown for the handsheets of comparative examples 2 to 4. This means that one can draw a virtual line through the dot for example 6, respectively, comparative example 9, which is parallel to the line formed by the dots for comparative examples 2 to 4. By doing so, it becomes obvious that at a given ash content, a higher breaking length can be achieved for handsheets containing polysilicate coated Calopake®F (example 6) compared to handsheets containing only Calopake®F (comparative examples 2 to 4) and also compared to handsheets containing a mixture of Calopake®F and polysilicate (comparative example 9). Or in other words: the same breaking length can be achieved at lower ash content for handsheets containing polysilicate coated Calopake®F compared to handsheets containing only Calopake®F or a mixture of Calopake®F and polysilicate.

The correlation of breaking length and ash content for the handsheets of example 7 to 10 and comparative examples 5 to 8 is shown in FIG. 2.

FIG. 2 shows that at a given ash content, a higher breaking length can be achieved for handsheets containing polysilicate coated Albaca®HO (example 7), Albaca®LO (example 8), Syncarb F0474 (example 9), respectively, Miconapaque HB (example 10) compared to handsheets containing the respective calcium carbonate without polysilicate coating (comparative examples 5 to 8). Or in other words: the same breaking length can be achieved at lower ash content for handsheets containing polysilicate coated Albaca®HO, Albaca®LO, Syncarb F0474, respectively, Miconapaque HB compared to handsheets containing the respective calcium carbonate without polysilicate coating.

The correlation between Scott bond strength and ash content for the handsheets of example 6 and comparative examples 2 to 4 and 9 is shown in FIG. 3, and for the handsheets of example 7 to 10 and comparative examples 5 to 8 is shown in FIG. 4. Here it can also be assumed that the correlation of Scott bond strength and ash content of the handsheets of examples 6 to 10 and comparative examples 5 to 9 behaves in analogy to the correlation of Scott bond strength and ash content as shown for the handsheets of comparative examples 2 to 4. This again means that one can draw a virtual line through the dot for the respective example, which is parallel to the line formed by the dots for comparative examples 2 to 4. When comparing the parallel lines, it can be seen that the correlation between Scott bond strength and ash content is almost identical for handsheets containing only Calopake®F (comparative examples 2 to 4) or a mixture of Calopake®F and polysilicate (comparative example 9), but that at a given ash content, a higher Scott bond strength can be achieved for handsheets containing polysilicate coated Calopake®F (example 6), Albaca®HO (example 7), Albaca®LO (example 8), Syncarb F0474 (example 9), respectively, Miconapaque HB (example 10) compared to handsheets containing the respective calcium carbonate without polysilicate coating (comparative examples 5 to 8). Or in other words: the same Scott bond strength can be achieved at lower ash content for handsheets containing polysilicate coated calcium carbonate compared to handsheets containing only calcium carbonate or a mixture of calcium carbonate and polysilicate.

In summary: The use of polysilicate coated calcium carbonate leads either to paper of increased breaking length and Scott bond strength at the same ash content or to paper of lower ash content, i.e. containing a reduced amount of fibres and/or calcium carbonate at the same breaking length or Scott bond strength. Thus the ratio of breaking length/ash content of filled paper, and the ratio Scott bond strength/ash content of filled paper are improved.

EXAMPLE 11 Preparation of Compositions Comprising Aqueous Polysilicate Coated Calcium Carbonate Using Sulfuric Acid as Acid

To 200 L water (standard town water) 21 L of sodium silicate (N®sodium silicate from PQ corporation, 28.7% (w/v) SiO₂, 8.9% (w/v) Na₂O, density at 68° F.: 1.38 g/cm³) is added. After stirring to achieve a homogenous mixture sufficient concentrated sulfuric acid is added to reduce the pH of the sodium silicate from 11.3 to 9.5 at room temperature. The sodium silicate is allowed to react for 3 minutes prior to the addition of 50 L of dispersed ground calcium carbonate (GCC) slurry (Hyrocarb®90 sold by Omya, 60% by weight GCC). The calcium carbonate is mixed into the slurry well, as well as a further 29 L of water before the silicate starts to form a solid 3 dimensional matrix. The solid gel is broken down by the action of the mixer and the mixture is stirred for further 2 hours. After this time a grainy slurry is produced with visible particle in the range of 0.5 to 2 mm. 10 L samples of this material is taken and sheared for 10 minutes at 14,000 rpm using a Polytron Emulsifier to give a uniform viscous aqueous based polysilicate coated calcium carbonate. The polysilicate coated calcium carbonate is then ready to use. 

1. A process for the preparation of an aqueous composition comprising polysilicate coated mineral particles, wherein the process comprises subjecting an aqueous composition comprising gelled polysilicate and a mineral to shear of at least 5,000 rpm for at least 30 seconds.
 2. The process of claim 1 wherein the aqueous composition comprising gelled polysilicate and a mineral is obtainable by polymerization of a silicate in the presence of a mineral.
 3. The process of claim 2, wherein the polymerization of the silicate is initiated by adjusting the pH to a pH of below 10 by addition of an acid.
 4. The process of claim 2, wherein the polymerization of the silicate is performed at a temperature of below 50° C.
 5. The process of claim 2, wherein the polymerization of the silicate is substantially complete within 12 hours.
 6. The process of claim 2, wherein the amount of mineral is in the range of 5 to 20% by weight based on the weight of the aqueous composition comprising gelled polysilisate and mineral.
 7. The process of claim 2, wherein the amount of silicate is in the range of 0.01 to 10% by weight based on the weight of the aqueous composition comprising gelled polysilisate and mineral.
 8. The process of claim 2, wherein the amount of gelled polysilicate and mineral is in the range of 1 to 30% by dry weight based on the weight of the aqueous composition comprising the gelled polysilicate and mineral.
 9. An aqueous composition comprising polysilicate coated mineral particles, which is obtained by the process of claim
 1. 10. The aqueous composition of claim 9, which is characterized by a viscosity of at least 500 mPas (when measured at a concentration of 12% by dry weight polysilicate coated mineral particles based on the weight of the composition, using a Brookfield viscometer at 25° C. and 100 rpm, spindle 4, and the result is taken 30 seconds after the start of the test).
 11. The aqueous composition of claim 9, wherein the amount of mineral is between 40 and 99% by weight based on the weight of the polysilicate coated mineral particle.
 12. The aqueous composition of claim 9, wherein the amount of polysilicate is between 1 and 60% by weight based on the weight of the mineral.
 13. The aqueous composition of claim 9, wherein the size of the polysilicate coated mineral particles is between 1 and 1000 μm.
 14. The aqueous composition of claim 9, wherein the amount of polysilicate coated mineral particles is between 1 and 30% by weight based on the weight of the aqueous composition.
 15. Polysilicate coated mineral particles obtained from the aqueous composition of claim 9 by removal of water.
 16. Paper or paper board filled with the polysilicate coated mineral particles of claim
 15. 17. A process for the preparation of the paper or paper board filled with polysilicated coated mineral particles, which comprises the step of adding the polysilicate coated mineral particles of claim 15 to a cellulosic suspension prior to drainage of the cellulosic suspension on a wire, where a web is formed, which is subsequently dried.
 18. A method for improving the tensile strength and internal bond strength of filled paper or paper board with respect to the basis weight of the filled paper or paper board, which method comprises adding the polysilicate coated mineral particles of claim 15 to a cellulosic suspension prior to drainage of the cellulosic suspension on a wire, where a web is formed, which is subsequently dried.
 19. (canceled)
 20. A process for the preparation of the paper or paper board filled with polysilicated coated mineral particles, which comprises the step of adding the aqueous composition comprising the polysilicate coated mineral particles according to claim 9 to a cellulosic suspension prior to drainage of the cellulosic suspension on a wire, where a web is formed, which is subsequently dried.
 21. A method for improving the tensile strength and internal bond strength of filled paper or paper board with respect to the basis weight of the filled paper or paper board, which method comprises adding the aqueous composition comprising the polysilicate coated mineral particles of claim 9 to a cellulosic suspension prior to drainage of the cellulosic suspension on a wire, where a web is formed, which is subsequently dried. 